Printed circuit board assemblies are used in computers, communications equipment, televisions and many other product. In a typical printed circuit board assembly, many electrical components are attached to the top and bottom surfaces of a printed circuit board (PCB). Since the electronics manufacturing industry is highly competitive, it is important to maximize the through-put of processing PCB assemblies and to securely attach the electrical components to the PCB.
Manufacturing PCB assemblies involves many processes, one of which is through-hole mounting packaged components and passive components to PCBs. To through-hole mount a component to a PCB, the pins of the component are positioned in sockets in the PCB, and then the PCB assembly is passed over a solder-wave machine that flows a wave of molten solder against the bottom side of the PCB. As the PCB assembly passes over the solder-wave, a portion of the solder connects the through-pins of the component to the sockets of the PCB, while the rest of the solder is blown off of the PCB to electrically isolate the separate pins and sockets.
Conventional solder-wave machines generally have a solder reservoir for holding a pool of molten solder, a nozzle for forming and directing the solder-wave, and a number of slide-plates positioned over the molten solder pool and adjacent to the top of the nozzle to protect the molten solder. A number of nitrogen defusers are generally positioned under the slide-plates to form an inert barrier on top of the molten solder pool that reduces the oxidation of molten solder. The slide-plates may be adjusted in two dimensions to vary the size of the opening for the nozzle, and the nozzle may be adjustable in three dimensions to vary the height and orientation of the nozzles to form the solder-wave at a desired height and location. Most solder-wave machines also have an air-knife positioned downstream from the nozzle to blow excess solder off of the PCB assembly and thus electrically isolate the soldered contacts from one another. The slide-plates, nozzle and air-knife are typically arranged along a conveyor path that is inclined at an angle of approximately 6.degree. to enhance the flow of solder off of the back side of the PCB assembly. It will be appreciated that the nozzle may be a chip-wave nozzle to form a turbulent wave or a contour-wave nozzle to form a smooth, laminar-wave. Additionally, the solder-wave machine may have both a chip-wave nozzle and a contour-wave nozzle aligned along the conveyor path to perform both functions in a single solder-wave machine.
In typical PCB assembly manufacturing lines, molten solder may splatter over the slide-plates and an oxidized layer of solder ("dross") may form on top of the molten solder pool. The layer of dross is generally scraped off of the molten solder pool on a daily basis, and the solder-wave machine is generally disassembled and cleaned thoroughly on a weekly basis. For example, to clean a solder-wave machine, the side-plates, nozzles and diffusers are generally removed from the solder reservoir so that dross and other matter may be cleaned from the individual components. Accordingly, the solder-wave machine must be reassembled before additional PCB assemblies can be processed.
Reassembling solder-wave machines, however, is difficult and time-consuming because the nozzles, side-plates and air-knife must be properly aligned to accurately contact the solder-wave with the backside of the PCB assemblies. For example, to cover the backside of the PCB assemblies with molten solder, the nozzles must be accurately positioned with respect to a plane through which the PCB assemblies travel over the solder-wave machine. The nozzles, however, cannot be positioned above the travel plane of the PCB assemblies because the nozzles would then interfere with the PCB assemblies during processing. Additionally, the side-plates must be positioned to cover the solder reservoir without interfering with the drainage of the solder-wave or the PCB assemblies, and the air-knife must be positioned where it can blow off excess solder without interfering with the PCB assemblies. Since the travel plane of the PCB assemblies is generally at a slight angle (e.g., 6.degree.), it is difficult to properly position the nozzles, side-plates and air-knife in proper alignment to perform their functions without interfering with the PCB assemblies.
One conventional process for aligning the components of a solder-wave machine is to use a ruler to measure the desired locations of the nozzles, the side-plates and the air-knife. However, the cleaning and reassembling processes take place while the solder-wave machine is typically very hot (e.g., approximately 480.degree. F.) because it may take over a day to cool the molten solder pool and approximately 8 hours to reheat the molten solder. Additionally, dross is highly toxic so it is undesirable to touch or otherwise spread any dross outside of the solder-wave machine. Therefore, aligning the components of a solder-wave machine with a ruler is very difficult and time-consuming.
Another technique to align the components of a solder-wave machine is to run a glass plate through the solder machine in the travel plane of the PCB assemblies to determine whether the solder-wave contacts; the surface of the glass plate. In this alignment technique, an operator observes the coverage of the solder-wave through the glass plate to see if the solder-wave contacts a sufficient surface area on the bottom side of the glass plate. The glass-plates provide a go/no-go indication of whether the height of the solder-wave is sufficient to cover the PCB assemblies, but they do not indicate the position of any low points along the width of the solder-wave or the distance that such low points deviate from the travel plane of the PCB assemblies. Thus, glass plate techniques simply provide a go/no-go test before processing a run of PCB assemblies.
In light of the difficulties of aligning the components of a solder-wave machine with a ruler or a glass plate, the components of a solder-wave machine are commonly aligned by "eye." However, "eye-balling" the alignment of the components may be inaccurate because it is difficult to align the components of the solder-wave machine along an inclined path. Moreover, the desired minimum clearance between the travel plane of the PCB assemblies and any component of the solder-wave machine is approximately 0.3". Thus, it is difficult to eyeball the precise locations of the components so that they are close enough to operate on the backside of the PCB assemblies without interfering with the PCB travel plane.